Europa's Hydrogen Corona in a Large Set of HST Lyman-Alpha Images

Sammanfattning: Far-ultraviolet (FUV) spectral images of Jupiter's moon Europa were obtained by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) on 20 occasions between the years 1999 and 2015. In this thesis these data are analyzed to look for Lyman-alpha emissions from a hydrogen corona. This hydrogen corona was recently discovered in absorption, also from HST Lyman-alpha images but with Europa in transit of Jupiter, and the aim of this study is to confirm the existence of the corona also in emission. Europa's thin atmosphere is dominated by molecular oxygen, mainly produced by radiolysis and sputtering of the icy surface. Atomic hydrogen, the main target for this study, is produced by sputtering from the surface and the dissociation of H2 and H2O. It quickly escapes the gravity of Europa. To study the hydrogen corona in the spectral STIS images the data need to be processed to remove the other Lyman-alpha contributions to the image. These other contributions include emissions from the geocorona, emissions from the interplanetary medium (IPM), dark current in the detector and sunlight reflected from the surface of Europa. To estimate the contribution to the image from the hydrogen corona, a basic model of the expected emissions from the corona is developed. By fitting this model to the processed STIS data values of the hydrogen density and the surface Lyman-alpha albedo of the moon are obtained. The results confirm the presence of a hydrogen corona, with varying densities between the different observations but generally about twice as large as the results from the previous study. The uncertainty for the results is however large and there is a clear correlation between hydrogen density and background level in the image, for which the reason is poorly understood. No hemispheric variability or connections to the true anomaly of the moon are found, but the hydrogen density seems to be increasing during the time of the observations. The results for the albedo is consistent with previous results, indicating a lower albedo on the leading than on the trailing hemisphere.